# Maximum current,

Discussion in 'Electronic Design' started by Danny, Oct 4, 2003.

1. ### DannyGuest

Hi

Could anyone tell me how to calculate the maximum current that a wire
carry, presumably a formula for the heating effect in relation to its
cross-sectional area? As also i would assume it brings in the thermal
coefficient.

Dan

2. ### Jeroen VriesmanGuest

For high currents it's not only heating.

If you got two wires next to each other with opposite currents, the
magnetic field can have enough force to split it into two separate wires.

skin effect, the electrons move on the surface of the wire, at higher
frequencies this effect is stronger.

Once a wire starts to heat, it will have more resistance, if the current
doesn't get lower when resistance get higher (is you circuit a current
source?) you will reach the point of no return right after the wire starts
to heat.

So a "complete formula" is kind of hard, thermal resistance of your
isolation and ambient temperature also play a part.

If you could define your circuit, and define the conditions, it's easyer to
make a formula.

Cheers,
Jeroen.

joenix at gmx dot net

3. ### DannyGuest

Hi

Well it is a question build upon many different circuit ideas, not
really a specific one, they are all, of sort, 12V automotive
applications, which currents upto about 10A at maximum, and as low as 500mA.

So if i needed say a wire to run a 5A bulb at 12 V, what clasification
of wire would i need. Is there like a rule of thumb that most people use
to classify current against wire type (like a table of ideal thicknesses
etc)?

Dan

4. ### Frank PickensGuest

Take a look at this site.
fp
http://www.powerstream.com/Wire_Size.htm

5. ### Jeroen VriesmanGuest

For only 10 amps DC, I wouldn't worry too much, just take a wire of 1mm^2
and your're done.

6. ### Fritz SchlunderGuest

Tables showing ampacity for wire sizes shouldn't be taken too literally, but
to give you an idea of what might be reasonable you might play around with
some AWG wire calculators like this one:

http://www.geocities.com/CapeCanaveral/Lab/9643/awg.htm

In many low voltage applications (of which automotive is a good example) the
wire current density is more limited by total voltage loss rather than the
maximum dissipation capability of the wire. As a result you as the
engineer get to decide how much voltage loss is appropriate in any given
application to determine maximum wire resistance and then wire size.

7. ### Ian StirlingGuest

Also, depends on in what condition you want the wire afterwards.
You can pass many thousands of amps through a wire thinner than a
hair, but after a few nanoseconds it'll be expanding outwards at several
kilometers per second...

8. ### Ian StirlingGuest

Also, you probably don't want to spec wires that can safely pass only
500ma, in an automotive enviroment.
Unless they are very well supported, they will be rather fragile, especially
when installing.
1mm^2 is a pretty good minimum.

What you probably want is to figure out the lowest voltage you find
acceptable at the bulb (maybe 11.75V when the wire is connected to
12V) and choose the wire based on that. This example gives a .25V drop
across the wire. At 5A this gives a resistance of .25V / 5A = 0.05
ohms. Choose a wire that will have that resistance or less at the
length (both directions, unless you use the chassis as the return)
from the source to the bulb.

10. ### N. ThorntonGuest

Hi

Car electrics aren't 12v, they are nearer 14v.
Also a fraction of a volt can make a significant difference to light
output on a [nominally] 12v system, and that of course affects safety
and driver comfort. So seeing what wire you can get away with probably
isn't the way to do it in this case.

IIRC around 3mm2 is commonly used for headlights.

Regards, NT